Replying to this separately instead of editing my other post, because this is a fantastic question.
Pub DotA and pub League are two very different beasts; pretty much every League player from whatever level jungling becomes viable (I haven't leveled an account since S1) understands that League has five hard-defined roles (Top, Mid, Jungle, ADC, Support) and most champions tend to pigeonhole into those roles. Obviously, there are tons of exceptions (Kha'zix can play three roles pretty effectively, Jungle/Tops tend to swap etc), but it's somewhat static. In ranked queues, people will call their roles and pick champions based on that grouping of roles most of the time.
In DotA, much like League, heroes are defined by their role. The cool thing is, pretty much any hero is suited for pretty much any role (again, with some obvious exceptions). Even though this is the case, almost every hero has a role they're -expected- to fill due to their skill progression and stats. The 1-5 System is really used to show in a perfect world who would be getting farm priority for both gold and experience. A 1 would be your Doublelift ADC; someone who want to protect at all costs and shovel gold into. 5 would typically be a pure-support hero (think S3 Sona) who spends 90% of their gold on wards / regen items for their carries / dusts etc - most of the time you'll rarely get anything apart from Boots and maybe a Bracer.
Here's a decent writeup on DotA roles:
http://dota2.gamepedia.com/Role
